Date of Completion

8-17-2017

Embargo Period

2-13-2018

Keywords

Ferroelectrics, superlattices, dielectric properties, pyroelectricity, thin films

Major Advisor

S. Pamir Alpay

Associate Advisor

Avinash M. Dongare

Associate Advisor

Serge M. Nakhmanson

Associate Advisor

Mark Aindow

Associate Advisor

Seok-Woo Lee

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Artificial ferroelectric (FE) heterostructures show unique electrical properties compared to bulk and single-crystal and thin film FEs. The internal built-in electrical fields due to heterogenoeuos nature of the structure gives rise to unique electrical properties. Dielectric and electrothermal (pyroelectric and electrocaloric) properties of such multilayers and superlattice (SL) heterostructures are investigated using a non-linear thermodynamic model. The underlying reasons for the enhancement of electrical properties of FE heterostructures are analyzed based on the strain and built-in electrostatic fields that could be used as design parameters under realistic processing conditions of such materials. It is shown that the choice of multilayer/substrate pair, processing/growth temperature, and relative layer fraction of ferroelectric could be tailored to enhance dielectric and electrothermal properties of FE multilayers. For instance, large tunabilities (90% at 400 kV/cm) are possible in carefully designed barium strontium titanate-strontium titanate (STO) and lead zirconate titanate (PZT)-STO even on silicon for which there exist substantially large in-plane strains. It is also possible to obtain enhanced electrocaloric response from multilayers. 0.75·barium titanate-0.25·PZT and 0.35·STO-0.65·PZT bilayers show ~120% and 65% increase in electrocaloric response, respectively, compared to PZT films on Si for ΔE=500 kV cm-1. It is possible to obtain enhanced dielectric and pyroelectric response from PZT-STO SLs, especially with increased number of repeating unit for a fixed thickness. It is possible to avoid domain splitting in the FE and obtain enhanced properties with reduced overall transition temperature. For example, small bias pyroelectric coefficient of 8-unit symmetrical SLs is ~55% higher than that of a zero bias PZT monolayer reaching 0.045 µC cm-2 K-1.

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